Steam raising plant which uses pulverised coal requires igniters for igniting the main burners, and the igniters may also be used, either alone or in conjunction with other burners, for providing supplementary energy for warm-up and flame stabilisation under reduced load conditions. The igniters have traditionally been fueled with gas or oil, and this has added significantly to both the capital and operating costs of the plant.
Attempts have been made to obviate the need for gas and oil igniters and to provide igniters that are fueled with pulverised coal. (P.R. Blackburn) U.S. Pat. No. 4,089,628 discloses an elementary burner arrangement in which an electric arc heated high velocity oxidising gas is used to ignite pulverised coal. The coal is delivered to the burner in an air stream and the coal-air mixture is contacted by a jet of the hot oxidising gas in a combustion chamber region of the burner. The hot gas jet is maintained in contact with the coal-air stream until there is sufficient ignition energy to ignite the pulverized coal, although the jet might be sustained after ignition has occurred in order to stabilise burning.
It is clear that this elementary igniter could not be made to work other than under optimum conditions and, thus, a different igniter design would need be created to meet different flow rate or coal-air mixture requirements and for burning different types of coal.
(D.A. Smith et al.) U.S. Pat. No. 4,221,174 discloses an igniter which also has been designed for direct ignition and combustion of pulverised coal. The igniter includes a source of pressurised air which is injected into a pulverised coal-air fuel stream at periodic intervals to create an air-to-coal weight ratio which varies cyclically with time and which, therefore, provides optimum conditions for ignition and flame propagation during a part of every cycle. Ignition is effected by a high energy spark which is excited at a rate greater than the rate of variation in the air-tc-coal weight ratio, and ignition occurs when optimum conditions exist. However, a significant feature of the device is that the source of ignition is discontinuous and this tends to reduce its reliability.
(M.E. Smirlock et al) U.S. Pat. No. 4,228,747 and (D.A. Smith et al.) U.S. Pat. No. 4,241,673 also disclose igniters which are intended to effect direct ignition of pulverised coal. These igniters both employ retractable electric spark producing mechanisms which are used only to initiate ignition of the pulverised coal, but they are otherwise somewhat similar to the more elementary igniter which is disclosed in U.S. Pat. No. 4,089,628.
One feature which is common to the igniters disclosed in all of the above referenced patents is that the air-entrained coal is delivered to a combustion zone of the igniters by way of a single channel and, thus, the igniting mechanism (be it in the form of a gas torch or a spark generator) is required to effect instantaneous combustion of the full or normal coal supply to the igniter.